The present invention relates to a surface light source device for obtaining a planar light source by guiding the light from a point or linear light source, and a liquid crystal display device using this surface light source device.
As an illuminating device such as a backlight of a liquid crystal display device, a surface light source device which guides the light from a point or linear light source by using a light guide means and forms a planar light source of a certain area is used.
Some surface light source devices used as, e.g., a backlight of a liquid crystal display device have a light source unit arranged in a side portion and output the light from the light source from its surface. Surface light source devices of this sort are called side lamp type surface light source devices.
One conventionally known side lamp type surface light source device uses a planar light guide plate, and a light source unit. This light source unit comprises a straight-tube fluorescent lamp or an LED array formed by arranging a plurality of LEDs (Light Emitting Diodes) and is arranged to oppose one end face of the light guide plate.
This surface light source device is used such that its front surface, i.e., the surface of the light guide plate faces the incident surface of an object to be illuminated, e.g., a liquid crystal display panel. The incoming light from the light source unit enters the light guide plate from its end face, guided by the light guide plate, and exits from its surface.
In the above conventional surface light source device, however, the light guided in the light guide plate exits from the surface of the light guide plate in various directions. Consequently, a large amount of light exits at very small angles with the surface of the light guide plate and uselessly scattered around. This results in an insufficient quantity of light being output in a frontal direction and its nearby directions, so it is impossible to obtain a high frontal luminance (luminance observed in the frontal direction). Also, the luminance distribution of the exit light is nonuniform.